Eosinophi1 (EO) phagocytes destroy parasites but can also damage host tissue. Although allergies are the most common example of EO-mediated pathology, the hypereosinophilic Syndrome (HES) is the most striking. HES is a systemic hematologic disorder characterized by multi-organ system involvement, preeminently a characteristic, usually lethal form of heart disease. Eosinophilic heart disease (EHD occurs in a variety of hypereosinophilic states, irrespective of their cause. Endocardial and myocardial deposition of EO specific granule proteins, of which eosinophil peroxidase (EPO) is the most abundant, is proposed to mediate cardiotoxicity in EHD. Despite the abundance of EPO and the vigor of the EO respiratory burst, little is known about the contribution of EPO-mediated oxidative damage to the pathology of eosinophilic inflammatory states. We find that three unusual substrates - bromide (Br-), nitrite (NO2-), and thiocyanate (SCN-) - compete for oxidation by EPO in physiologic fluids in the presence of H202, yielding, respectively, HOBr, NO2., and HOSCN. The relative toxicity of these oxidants for human cells is HOBr> NO2>> HOSCN; yet EPO preferentially oxidizes SCN > NO2 - > Br-. We hypothesize that SCN "buffers" against generation by EPO of the more cytotoxic N02- and Br- based oxidants and consequently serum SCN- levels, which are dietarily determined, may modulate EPO toxicity. The overall goal of our proposed work is to examine the hypothesis that that EPO-generated oxidants impose damage in a substrate-determined manner to mammalian cells and tissue that contributes to the pathogenesis of EHD, which we propose as a paradigm for any organ damage occurring in eosinophil inflammatory states. The first specific aim is to test the hypothesis that whereas HOBr and N02 cause necrotic cell death by reacting with membrane components to destroy membrane integrity, HOSCN imposes sulfhydryl-targeted intracellular oxidative stress that can influence tissue factor gene expression and induce apoptosis. The second specific Aim is to test the hypothesis that an interleukin-5 (IL.5) transgenic mouse line will develop progressive functional, anatomic, and histologic manifestations of EHD with a parallel accumulation of EPO-specific protein oxidative damage as assessed by newly developed sensitive, substrate-specific amino acid "biomarkers." The third specific aim is to test the hypothesis that EPO contributes to the pathogenesis of EHD in the IL.5 transgenic mouse line. We will compare EHD severity, longevity, and EPO biomarker levels in the IL-5 transgenic line with that of an IL-S transgenics crossbred with an EPO 'knockout" line. The fourth specific aim is test the hypothesis that increasing serum SCN- inhibits, and decreasing SCN- promotes, EPO-mediated protein oxidant damage and the severity of EHD in IL-5 transgenic mice. These studies, if successful, will establish a murine model for HES and EHD, prove a mole for EPO-mediated oxidant damage in the pathogenesis of EHD, and suggest a simple strategy fur its treatment, (i.e., dietary supplementation with inexpensive SCN-), that could be applied even in impoverished communities as well as against more common allergic diseases, such as asthma.